Actuators are useful in a number of devices including but not limited to fluidic and pneumatic control valves. Fluidic and pneumatic control valves are commonly used as components of power control and distribution circuits for stationary and mobile power equipment, as well as industrial process control systems.
Fluid control valves can be categorized into valves with discrete positions, such as open, neutral (locked) or closed, or proportional valves where the controlled variable, such as flow, is proportional to the magnitude of the control or input signal. A number of valve types are commercially available, including spool valves, butterfly valves, and poppet valves.
In spool valves, spool control is typically achieved through moving a solenoid core (a spool) through electromagnetic coil actuation. Spool valves can be of the 2-position, 3-position, or proportional type (independent of the number of ports connected to the valve). In a 2-position valve, the spool (or other control device) is in one of two extreme positions during operation. When the coil is energized, the spool is moved to either the completely closed or the completely open position, without controlling the position as the spool moves from the one position to the next. If the valve has latching capabilities, valve control is maintained in the most recent condition without the application of continuous external power.
A 3-position valve adds a neutral central position to the 2-position layout. This achieves functionality such as driving a hydraulic ram forward, backwards, or locking it into place at the third, neutral position. In some environments, the ideal flow control valve is a proportional control valve, where the position of the control device can be controlled accurately to achieve not only open, closed or neutral but also by varying degrees of open and closed.
U.S. Pat. No. 6,474,353, incorporated herein by reference, describes an on/off latching solenoid control valve, with a neutral position to lock all ports, typifying it as a 3-position valve. Electromagnetic coils are used to shuttle the valve spool between the two extreme spool positions. Coil magnetization provides the force to move the spool into the desired position, while at the same time magnetizing the spool. Remnant magnetization in the spool causes the spool to latch onto the energized coil body, thus retaining spool position when the coil is de-energized. This latching effect produces a no-power hold condition that limits electrical power consumption during spool movement. Detachment from the coil body is achieved by sending a small current through the appropriate coil to unlatch the spool to enable springs to move the spool into a neutral position.
However, valves controlled by a solenoid have several disadvantages. Solenoid actuation is limited in terms of the time required to open or close the spool (also referred to as the actuation frequency). Actuation frequencies are generally low. The inertia of the spool and the solenoid core limit the spool traveling time. In order to reduce the traveling time, the movable mass must be reduced.
As an alternative to solenoid actuation, spool movement is also achieved through the use of solid-state actuation materials such as piezoelectric, electrostrictor, and magnetostrictive materials. These actuation materials produce strain directly in response to an externally applied electric or magnetic field. However, use of these materials has its own disadvantages. For example, most solid-state actuation materials produce very low levels of strain. As a result, these materials do not find wide application in valves where large control device displacements are required.
Overall, currently known actuator systems tend to cover the extremes of stroke-force domain with either very little stroke at high force, or high stroke at low force levels. For general industrial actuation applications including many known valves, servo-flap drivers and injectors, it is necessary to devise complex stroke gaining mechanisms to improve the stroke capability, which comes at added cost and complexity.
Therefore, it would be advantageous to have a large strain, solid-state actuator material system with high actuation frequency force, over a wide frequency band, with simple driving mechanisms, especially for use in valve systems.
One type of known material, namely magneto-active twinned material (MATM), sometimes also referred to as ferromagnetic shape memory alloy (FSMA), magnetic shape memory alloy (MSMA) or magnetically controllable memory alloy (MCMA), is a class of solid state active materials that satisfies the need for high frequency and large stroke actuation at appreciable force levels. The actuation is caused by a phase transformation in the material, based on twin boundary motion, which is driven by an externally applied stress or magnetic field.
Such materials provide large strains in response to external stimuli. U.S. Pat. No. 5,958,154, incorporated herein by reference, describes a general class of magneto-active twinned materials that provide large actuation strains at high actuation rates based on an externally applied magnetic field. This class of materials exhibits a twinned martensitic crystal structure below a characteristic phase transformation temperature. The magnetocrystalline anisotropy energy levels of the material allow for twin boundary motion through the application of an external magnetic field. The twin boundary motion allows switching of the twins in the structure of the material, resulting in a macroscopic straining of the material.
However, typical known actuator arrangements utilizing such materials also utilize mechanical devices, such as springs, to provide mechanical stress to return the actuator material to its original condition. In such a system, power needs to be supplied continuously, for instance, to keep a valve open. This continuous power application wastes electrical power and generates heat in the valve, thus creating drawbacks and failing to take full advantage of the properties of the material in the context of position control actuator systems.